At present, the development of mobile or nomadic terminals such as portable cellular phones, smart phones, PDAs standing for “Personal Digital Assistant” as well as the development of multimedia portable data terminals designed to receive television or related services, is growing steadily, using applications such as WIFI (Wireless Fidelity), WIMAX (Worldwide Interoperability for Microwave Access), DVB-T, DVB-H (Digital Video Broadcast) or other similar applications.
In order to receive these types of applications, the terminals are fitted with antennas, more specifically with antennas operating in the UHF frequency band, namely the band covering 470 MHz to 862 MHz frequencies, or in higher frequency bands.
In fact, a considerable bandwidth, the lowest frequency of the UHF band and compactness are major constraints for the design of an antenna that can be integrated in nomadic or mobile terminals.
Among the antennas that can be integrated, there are in particular planar antennas constituted by a radiating slot. However a radiating slot in linear shape etched in a ground plane presents a length modulo λg/2 where λg is the guided wavelength in the slot at the operating frequency. Thus, as represented in FIG. 1, with a rectilinear slot 1 etched in a ground plane 2 produced on a known dielectric substrate and fed at 3 either directly through a coaxial or by using the known technique of electromagnetic coupling described by Knorr, all of the field lines radiate in phase and are oriented in the same direction, as symbolized by the arrows F.
In a known fashion and as represented in FIG. 2 for a 2.4 GHz radiating slot, the orientation of the field lines is due to the current induced through the length of the slot, said currents being symbolized by the current vectors V through the length of the slot 1 of FIG. 2.
The design represented in FIG. 1 and FIG. 2 is the design of a 2.4 GHz radiating slot in a finished ground plane of a dimension of 111.2 mm×60.5 mm. In this case, the dielectric substrate chosen is the known substrate Rogers 4003, whose physical parameters are thickness 0.8 mm, permittivity ∈r=3.38 and loss tangent δ=0.0027.
In the case of FIGS. 1 and 2, the slot is excited by a microstrip line 3 short circuited at its extremity. This type of excitation obeys the conditions for coupling a microstrip line to a slot line as defined by Knorr (refer to article J. B. Knorr “Slot lined transition” IEEE Trans. Microwave Theory and Techniques, pages 548-554, May 1974). In this case, the characteristics of the slot are as follows:                slot length: 42.4 mm (˜λg/2),        slot width: 0.5 mm.        
As the person skilled in the art knows, this slot presents a non-negligible length, depending on the operating frequency, which makes this type of antenna difficult to integrate in a mobile terminal. Owing to this fact, in order to reduce the overall dimension and as shown in FIG. 3, it is a known practice to bend the strands 10a, 10b of the slot 10 into a spiral. However, as it will be explained in a more detailed manner hereinafter, the radiating efficiency of such a radiating slot decreases significantly.
In FIG. 3, we have shown a slot 10 etched in the ground plane 11 of a dielectric substrate. This slot 10 is fed in its middle portion 12 by a microstrip line, according to a Knorr type feed. This slot contains two strands 10a, 10b which have each one been noticeably folded into a rectangular shape open at the end of the strand. This specific shape of the strands 10a, 10b makes it possible to limit the total overall size of the antenna. In this case, the longitudinal dimension is reduced from 42.4 mm to 9.5 mm for a length of 8.05 mm in the perpendicular direction.
As represented in FIG. 4 which gives the efficiency according to the frequency respectively for an antenna in accordance with FIG. 1 and an antenna in accordance with FIG. 3, with the dimensions given above, a fall is noticed in radiating efficiency at 2.4 GHz which passes from around 95% to 50%. This is explained by the fact that when the strands 10a or 10b are bent, the field lines in the parallel parts of the antenna, as represented by the arrows F1 and F2 in FIG. 3, noticeably cancel each other out, which decreases the radiating efficiency of this type of antenna.